A technician sets up a small wireless network, but it appears to provide less coverage...

The CompTIA Tech+ Study Guide details that wireless performance and range are significantly affected by interference:

“Interference from other electronics and obstructions will lower the data rate. Radio frequency interference (RFI) from communications devices will degrade the signal. Microwave ovens and electrical motors are notoriously harsh on wireless signals. Solid objects, especially concrete block walls and steel, cut down on range as well.”

Hence, interference is the most likely cause for the reduced wireless coverage.

Verified Reference:

Chapter 8: Networking Concepts and Technologies — “Wireless Interference and Attenuation” (pp. 440–441).